Hailan Zhang

Characteristics of group velocities of backward waves in a hollow cylinder

It is known that modes in axially uniform waveguides exhibit backward-propagation characteristics for which group and phase velocities have opposite signs. For elastic plates, group velocities of backward Lamb waves depend only on Poissons ratio. This paper explores ways to achieve a large group velocity of a backward mode in hollow cylinders by changing the outer to inner radius ratio, in order that such a mode with strong backward-propagation characteristics may be used in acoustic logging tools. Dispersion spectra of guided waves in hollow cylinders of varying radii are numerically simulated to explore the existence of backward modes and to choose the clearly visible backward modes with high group velocities. Analyses of group velocity characteristics show that only a small number of low order backward modes are suitable for practical use, and the radius ratio to reach the highest group velocity corresponds to the accidental degeneracy of neighboring pure transverse and compressional modes at the wavenumber k = 0. It is also shown that large group velocities of backward waves are achievable in hollow cylinders made of commonly encountered materials, which may bring cost benefits when using acoustic devices which take advantage of backward-propagation effects.

Acoustic field in a cased well with a sectorial crossing channel

To study the possibility of detecting the crossing channel by using a traditional logging tool, acoustic field generated by a monopole source in a cased well with a crossing channel of various angles is simulated by 2.5-D finite difference method for the first time. Snapshots of normal stress and synthetic time-domain waveforms are displayed. The two-dimensional spectrum in wave-number and frequency domains is also calculated, where the influence of the channel is clearer than that in the waveforms. Numerical study demonstrated that a crossing channel can be detected and sized if its angle is greater than 30 degree, and larger offset and lower frequency source are favorable to detect and size the crossing channel.The present invention is related to a speech synthesizer which includes a sampled signal storing device storing therein a sampled signal and outputting the sampled signal in response to an input signal, and a speech signal synthesizing circuit electrically connected to the sampled signal storing device, receiving an operation signal, having the sampled signal outputted by the sampled signal storing device be repeatedly operated in response to the operation signal, and then outputting a speech synthesized signal, wherein a frequency of the operation signal is higher than that of the input signal to allow the sampled signal to be repeatedly operated during a single cycle of the input signal. The present invention proceeds a plurality of times of operation for each entry of data in the storing device so that the synthesizing performance of the present synthesizer can be improved without increasing the storage amount of the sampled signals.

Studies on phase and group velocities from acoustic logging

It is still argued whether we measure phase or group velocities using acoustic logging tools. In this paper, three kinds of models are used to investigate this problem by theoretical analyses and numerical simulations. First, we use the plane-wave superposition model containing two plane waves with different velocities and able to change the values of phase velocity and group velocity. The numerical results show that whether phase velocity is higher or lower than group velocity, using the slowness-time coherence (STC) method we can only get phase velocities. Second, according to the results of the dispersion analysis and branch-cut integration, in a rigid boundary borehole model the results of dispersion curves and the waveforms of the first-order mode show that the velocities obtained by the STC method are phase velocities while group velocities obtained by arrival time picking. Finally, dipole logging in a slow formation model is investigated using dispersion analysis and real-axis integration. The results of dispersion curves and full wave trains show similar conclusions as the borehole model with rigid boundary conditions.

Simulation of 2.5-dimensional borehole acoustic waves with convolutional perfectly matched layer

A 2.5-dimensional method using the PDE package of the commercial finite element software COMSOL Multiphysics is developed to simulate wave propagation in a borehole. The computation is conducted in the frequency wave-number domain. A convolutional perfectly matched layer is implemented to eliminate the reflections from artificial truncation boundaries. Waveforms obtained in time domain are in good agreement with analytic solutions in a special model, which proves the validity of the method. A numerical modeling example is presented to illustrate the capabilities of the method. It is shown that this method can be used to solve a variety of non-axisymmetric borehole acoustic wave propagation problems.

Phase Relation of Harmonics in Nonlinear Focused Ultrasound

The phase relation of harmonics in high-intensity focused ultrasound is investigated numerically and experimentally. The nonlinear Westervelt equation is solved to model nonlinear focused sound field by using the finite difference time domain method. Experimental waveforms are measured by a robust needle hydrophone. Then the relative phase quantity is introduced and obtained by using the zero-phase filter. The results show that the nth harmonic relative phase quantity is approximately (n — 1)π/3 at geometric center and increases along the axial direction. Moreover, the relative phase quantity decreases with the increase of source amplitude. This phase relation gives an explanation of some nonlinear phenomena such as the discrepancy of positive and negative pressure.

Guided modes with multiple zero-group-velocity points in fluid-filled cylindrical pipes

It is known that guided modes in isotropic hollow cylinders exhibit backward wave propagation with negative group velocity. And interferences between backward and forward waves generate zero-group-velocity (ZGV) resonances with a finite wavenumber but vanishing group velocity. These ZGV resonances can be applied for non-destructive evaluation (NDE) of hollow pipes. In this paper, influences of a fluid-loading on ZGV resonances in pipes are studied for the possible application of integrity inspections of oil transportation pipelines. From numerically simulated frequency-wavenumber spectra of axisymmetric guided modes, in addition to the backward mode with a single ZGV point, certain branches change the sign of their slopes for twice (i.e., two ZGV points in one branch). Such multiple ZGV modes might be caused by the strong repulsion between the backward mode with a single ZGV point that is propagating in the hollow pipe and a number of longitudinal modes in the fluid cylinder. It is found that, from wave structure analyses, ZGV points correspond to relatively large displacement amplitudes at the pipe’s inner and outer interfaces. It indicates that guided modes with multiple ZGV points can be sensitive to the surface features of fluid-filled pipes, which is useful for NDE application.

Wave propagation in a fluid-filled shell excited by a dipole source

Wave propagation in a fluid-filled cylindrical shell excited by a dipole source is investigated for the design of a new kind of calibration pit for dipole acoustic logging tools. Based on classical elasticity wave equations, phase and group velocity dispersion curves of each mode, excitation spectra and mode contributions to wave field in the shell are presented. The dispersion curves show that the lowest mode exists in the entire frequency range with the phase velocity smaller than shear wave velocity of the shell, higher-order modes have cutoff frequencies, below the cutoff frequencies these modes are non-propagating. Analyses on mode excitation spectra and their contributions suggest that the lowest mode has potential to be used in calibrating the velocity measurement accuracy of dipole acoustic logging tools quantitatively, provided that the contributions of this mode to the wave field dominating the first arrivals gathered by a specific dipole acoustic logging tool.

Simulation of focusing high intensity ultrasound by time reversal method

Numerical simulations are used to investigate the validation of using time reversal to focus sound beams in high intensity ultrasound. Time reversal is a method based on linear and lossless sound field. But the high intensity of HIFU sound field brings with high nonlinearity and body tissues brings with thermoviscous absorption. So it is necessary to investigate whether time reversal is feasible in focusing sound beams in HIFU, whether it is preferable to normal methods. Nonlinearity and absorption is considered in numerical simulation based on KZK nonlinear parabolic wave equation. A time-domain algorithm with two-dimensional finite difference is used to solve the equation. Layered biological tissue model is created. Numerical results with various parameters are calculated to investigate the influence of nonlinearity and absorption on time reversal results respectively. Moreover, results are calculated both using time reversal and phased array method, to compare the differences on beam focusing behavior. It is founded that thermoviscous absorption has less effect on time reversal, while the influence of nonlinearity is greater. It is shown that in the presence of nonlinearity and thermoviscous absorption in HIFU, the ability of time reversal to focus the sound beams is also robust. Comparison results are shown that time reversal method is more accurate than normal method in inhomogeneous media considering nonlinearity and thermoviscous absorption. So time reversal method has a great prospect to focus sound beams in HIFU.

NUMERICAL STUDIES ON THE ACOUSTIC FIELD GENERATED BY A DIPOLE SOURCE IN NONCIRCULAR PIPES

Acoustical fields generated by an acoustical dipole sources in noncircular pipes filled with fluids are calculated by using of a 2.5 D method. The calculation is firstly carried out in the frequency wavenumber domain using PDE mode in the software system of Comsol Multiphysics. The results are then transformed into the time space domain. The numerical results show that the noncircular pipe may provide useful information for testing and calibrating acoustic dipole logging tools.

Backward waves with double zero-group-velocity points in a liquid-filled pipe

Hollow cylinders often exhibit backward propagation modes whose group and phase velocities have opposite directions, and these exhibit a minimum possible frequency at which the group velocity vanishes at a nonzero wavenumber. These zero-group-velocity (ZGV) points are associated with resonant conditions in the medium. On the basis of ZGV resonances, a non-contact and laser ultrasound technique has been developed to measure elastic constants of hollow pipes. This paper provides a theoretical and numerical investigation of the influence of the contained liquid on backward waves and associated ZGV modes, in order to explore whether this ZGV technique is suitable for in-service non-destructive evaluations of liquid-filled pipes. Dispersion spectra and excitation properties have been analyzed. It is found that the presence of the liquid causes an increased number of backward modes and ZGVs which are highly excitable by a point source. In addition, several guided modes twice undergo a change of sign in the slopes of their dispersion curves, leading to two ZGV points. This phenomenon of double ZGVs in one backward wave, which is caused by strong mode repulsions, has not been found in isotropic hollow cylinders, but it can be observed in a fluid-filled thin-walled pipe.